It is quite important for a semiconductor laser to oscillate at a single wavelength in its practical use. Therefore, a variety of devices have been suggested to cause oscillation of a semiconductor laser at a single wavelength. Among those, there is a DFB (distributed feedback) laser in which diffraction grating has a constant period in the resonator length direction at the neighborhood of the active layer inside the cavity. In this DFB laser, as a result of the diffraction grating, only light having a wavelength which satisfies the Bragg reflection condition resonates within the waveguide structure, thereby producing a single wavelength oscillation. In the DFB laser having such a constant period diffraction grating, however, there is actually no oscillation at the single Bragg wavelength, oscillation occurs at two wavelengths shifted toward the longer wavelength side and shorter wavelength side from the Bragg wavelength, respectively. In order to force the DFB laser to oscillate at a single wavelength, it is required to provide a .lambda./4 shift region for shifting the phase of the light guided in the reverse direction at the central portion of the diffraction grating.
FIGS. 2(a) to 2(f) show cross-sectional views of a prior art method for producing a .lambda./4 shift type diffraction grating. In the Figures, reference numeral 1 designates a semiconductor substrate, reference numeral 2 designates an insulating film, and reference numerals 3, 4, 5, and 6 designate a first resist, a second resist, a third resist, and a fourth resist, respectively.
The production process will be described.
First of all, a first resist 3 is deposited on a semiconductor substrate 1, and as shown in FIG. 2(a), a diffraction grating pattern is produced utilizing an interference exposure method, i.e., by exposure to a pattern of interferance fringes.
Next, as shown in FIG. 2(b), an insulating film 2 comprising such as SiN.sub.x is deposited on the substrate 1 and the diffraction grating pattern of the first resist 3 by a low temperature growth method such as electron cyclotron resonance (ECR) plasma chemical vapor deposition (CVD).
Next, as shown in FIG. 2(c), almost half of the length in the resonator length direction of the substrate 1 is covered by a third resist 5, and the insulating film 2 which is not covered by the third resist 5 is removed by wet etching using a fluorine-containing etchant of or plasma etching using CF.sub.4. Thereafter, the substrate 1 is etched using the pattern comprising the first resist 3.
Next, as shown in FIG. 2(d), the third resist 5 is removed, and the insulating film 2 above the first resist 3 is removed by selectively etching it, thereby exposing the first resist 3 and leaving the insulating film 2 between the diffraction grating pattern of the resist 3. Herein, the characteristic that the etching rate of the insulating film 2 on the resist 3 is higher than that of the insulating film 2 on the substrate 1 is utilized.
Next, as shown in FIG. 2(e), the exposed diffraction grating of the first resist 3 is removed, and a fourth resist 6 is deposited on the portion of the diffraction grating of the substrate which is produced by etching in the process of FIG. 2(c), and the other portion of the substrate is etched utilizing the pattern of the insulating film 2.
Thereafter, the insulating film 2 and the fourth resist 6 are removed, thereby to complete a diffraction grating provided with a .lambda./4 shift region at the central portion thereof as shown in FIG. 2(f).
The prior art .lambda./4 shift type diffraction grating is produced as described above, and in the process of FIG. 2(b), an insulating film 2 is deposited in a low temperature process in order to prevent the deterioration of the first resist 3. Although the low temperature deposition is used, the deterioration of the first resist 3 at the production of the insulating film 2 cannot be sufficiently prevented, whereby the resist hardens and cannot be easily removed later. Therefore, a load is applied to the substrate in the resist removing process, and the quality of the produced diffraction grating is deteriorated.